Like, try the "Ripple Tank Applet" and select "Zone Plate" using the Setup: pulldown in the upper right. Wow! So that's how Fresnel zone plates work!

Or try the "2D Wave Applet" and try "Setup: multiple slit" diffraction and play with the slit width and separation. What does it look like with polychromatic (multi-colored) light? Ever wonder why your credit card hologram is rainbow-y?

And don't even get me started on the 3-D magnetostatic fields applet...